Thermal image-fixing apparatus with a variable resistance heater roller

ABSTRACT

An image-fixing apparatus including a presser roller; a heater roller held in contact with the presser roller to provide a nipper for nipping a recording sheet bearing an unfixed image thereon, an electric resistance of the heater roller decreasing as respective contact areas of the heater and presser rollers which areas define the nipper increase; a supplying device which supplies an electricity to the heater roller so that the nipper generates a thermal energy to thermally fix the unfixed image on the recording sheet; a measuring device which measures the electricity supplied from the supplying device to the heater roller; and a sheet-jamming detecting device which detects a jamming of the recording sheet at the nipper, based on the electricity measured by the measuring device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thermal image-fixing apparatus whichthermally fixes an unfixed image such as a toner image on a recordingsheet.

2. Related Art Statement

There is known a thermal image-fixing device which thermally fixes anunfixed toner image on a recording sheet in a printer or copier of anelectrophotographic type. The image-fixing device includes a heaterroller and a halogen lamp. The heater roller is provided by a hollowcylindrical roller formed of a heat-conductive material such as aluminumor stainless steel. The heater roller is rotatable around the halogenlamp fixed in the hollow space of the roller, so that the heater rollerreceives, from the halogen lamp, an amount of thermal energy necessaryto fix the toner image on the recording sheet. The image-fixing devicefurther includes a presser roller formed of, e.g., a heat-resistantsilicone rubber. The presser roller cooperates with the heater roller toprovide a nipper for nipping, with a suitable pressing force, therecording sheet to which the toner image has been transferred.

The prior image-fixing device further includes a temperature sensorlocated adjacent the heater roller, and a control device which adjusts,based on the temperature detected by the sensor, an amount ofelectricity supplied to the halogen lamp and thereby controls the amountof heat generated by the halogen lamp, so that the surface temperatureof the halogen lamp is maintained at suitable values. The image-fixingdevice further includes a thermal fuse located adjacent the heaterroller. In the event that the amount of heat generated from the halogenlamp is not controllable because of the occurrence of some trouble withthe image-fixing device, the thermal fuse shuts off the supplying ofelectricity to the lamp, thereby preventing a fire or other safetyhazards.

In the prior image-fixing device, however, the halogen lamp heats (a)other portions of the heater roller than a portion thereof correspondingto the nipper where the heater roller contacts the recording sheet andthe toner image thereon; (b) bearings which rotatably support the heaterroller; and (c) ambient air around the halogen lamp. Thus, theimage-fixing device suffers from the following problems:

First, the bearings and other elements for supporting the heater roller,presser roller, temperature sensor, and thermal fuse must be producedusing a heat-resistant material such as a resin or a metal. Inparticular, the bearings for rotatably supporting the heater roller isrequired to possess a high resistance to heat. In addition, since theoverall image-fixing device is subject to high temperatures because ofthe heat generated from the halogen lamp, the image-fixing device mustentirely be covered with a heat-insulative member for protecting a userfrom being burnt when the user removes a recording sheet jammed at theimage-fixing device or the user's hand accidentally touches theimage-fixing device. Otherwise, the prior image-fixing device must bedisposed at a place where the user cannot touch the device. Thus, theconstruction of the image-fixing device is inevitably complicated, andthe overall size of the same increases.

Second, various elements associated with the image-fixing device must bespaced away from the device, so that those elements are prevented frombeing adversely affected by the heat generated from the device. Inaddition, heat-insulative members and/or electric funs must be providedbetween the image-fixing device and other devices such as aphotosensitive body, developing device, scanner, toner cartridge, etc.of the printer or copier. For this reason, the construction of theprinter or copier is complicated, and the overall size of the sameincreases.

Third, it takes a long time for the heater roller to have animage-fixing temperature at the outer surface thereof as a result ofconduction of the heat generated from the halogen lamp to the innersurface of the heater roller after application of electric power to theimage-fixing device. Thus, a long waiting or building-up time is neededbefore the heater roller is entirely heated to the image-fixingtemperature after the halogen lamp is turned on. This is inconvenientfor users who wish to start the image-fixing device as soon as possible.

The fourth problem is that the heat radiated from the heater roller isonly partly used to thermally fix the toner image on the recordingsheet. A considerable portion of the radiated heat is not used. Thus,the prior image-fixing device suffers from low energy efficiency.

The fifth problem is that in the case where a preheating of the heaterroller to a raised temperature for reducing the waiting or building-uptime is carried out after electric power is initially applied to theimage-fixing device and before an image-fixing operation is actuallystarted, the image-fixing device uselessly consumes electric power whileno image-fixing operation is carried out.

Sixth, the prior image-fixing device is composed of elements each havinga small thermal capacity, for improving the energy efficiency and/orreducing the waiting time. Therefore, there is a tendency that portionsof the heater roller other than a portion thereof contacting therecording sheet are raised to excessively high temperatures. It may beconsidered that if the cross-sectional area of the heater roller isdecreased for reducing the thermal capacity thereof, the thermalconductivity of the roller as taken in the longitudinal directionthereof is lowered, therefore heat does not easily conduct in thelongitudinal direction and tends to remain locally on the roller.

SUMMARY OF THE INVENTION

It is therefore a first object of the present invention to provide athermal image-fixing apparatus capable of detecting the occurrence ofjamming of various recording sheets having different width and/orthickness values.

The first object has been achieved by the present invention. Accordingto a first aspect of the present invention, there is provided animage-fixing apparatus comprising a presser roller which is rotatableabout a first rotation axis thereof; a heater roller which is rotatableabout a second rotation axis thereof parallel to the first rotation axisand which is held in contact with the presser roller to provide a nipperfor nipping a recording sheet bearing an unfixed image thereon, anelectric resistance of the heater roller decreasing as respectivecontact areas of the heater and presser rollers which areas define thenipper increase; a supplying device which supplies an electricity to theheater roller so that the nipper generates a thermal energy to thermallyfix the unfixed image on the recording sheet; a measuring device whichmeasures the electricity supplied from the supplying device to theheater roller; a sheet-data obtaining device which obtains sheet datarepresenting at least one of a width and a thickness of the recordingsheet; a memory in which is stored a relationship between the at leastone of the width and the thickness of the recording sheet and apermission range regarding the electricity; and a control device whichstops the supplying of the electricity from the supplying device to theheater roller, when the electricity measured by the measuring devicedoes not fall within the permission range corresponding to the at leastone of the width and the thickness of the recording sheet represented bythe sheet data obtained by the sheet-data obtaining device.

In the thermal image-fixing apparatus in accordance with the firstaspect of the invention, the control device stops the supplying of theelectricity from the supplying device to the heater roller, when theelectricity measured by the measuring device does not fall within thepermission range corresponding to the at least one of the width and thethickness of the recording sheet represented by the sheet data obtainedby the sheet-data obtaining device. In the case where the recordingsheet is jammed at the nipper provided by the heater and presser rollersand consequently the "effective" area of the nipper defined by therespective contact (or deformed) areas of the two rollers increase, theelectric resistance of the heater roller decreases, so that the electriccurrent of the electricity supplied to the heater roller may beincreased to an excessively high level to maintain the electric voltageof the electricity at a constant level. In this case, the control devicestops the supplying of the electricity from the supplying device to theheater roller, thereby effectively preventing various elements providedaround the present image-fixing device from being damaged by anexcessively great amount of heat generated from the heater roller. Inaddition, in the case where the memory stores the relationship whichprovides lower permission ranges for smaller widths of the recordingsheet, the control device may be adapted to stop the supplying of theelectricity for the smaller widths of the recording sheet, and not tostop it for the greater widths of the same, at the same measuredelectricity. Thus, the present image-fixing apparatus detects theoccurrence of jamming of various recording sheet having different widthand/or thickness values.

In a preferred embodiment in accordance with the first aspect of theinvention, the heater roller comprises: (a) a cylindrical resilientlayer which is formed of an electrically insulating and resilientmaterial, is concentric with the second rotation axis, and has aplurality of through holes formed through a thickness thereof; (b) afirst cylindrical electrode layer which is concentric with the secondrotation axis and has a plurality of electrodes each of which extendsinto a corresponding one of the through holes of the resilient layer;(c) a cylindrical resistant layer concentric with the second rotationaxis, the resistant layer and the first electrode layer sandwiching theresilient layer; and (d) a second cylindrical electrode layer concentricwith the second rotation axis, the second electrode layer and theresilient layer sandwiching the resistant layer, a portion of theresilient layer corresponding to the nipper being resiliently deformableso that a portion of the first electrode layer corresponding to thenipper is held in electric contact via the electrodes thereofcorresponding to the nipper with a portion of the resistant layercorresponding to the nipper, the supplying device supplying theelectricity to the heater roller through the first and second electrodelayers so that the portion of the resistant layer corresponding to thenipper generates the thermal energy to thermally fix the unfixed imageon the recording sheet. When the recording sheet is jammed at thenipper, the "effective" area of the nipper increases and the number ofthe electrodes of the first electrode layer corresponding to the nipperincreases, so that the electric resistance of the heater rollerdecreases.

In another embodiment in accordance with the first aspect of theinvention, the sheet-data obtaining device comprises a sheet-dimensionmeasuring device which measures, as the sheet data, the at least one ofthe width and the thickness of the recording sheet.

In yet another embodiment in accordance with the first aspect of theinvention, the control device comprises: control means for controllingthe supplying of the electricity from the supplying device to the heaterroller, so that the electricity measured by the measuring device fallswithin a control range fully covered by the permission range; andstopping means for stopping the supplying of the electricity from thesupplying device to the heater roller when the controlling of thecontrol means regarding the supplying of the electricity is noteffective in causing the electricity measured by the measuring device toremain within the control range and eventually the measured electricityhas deviated from the permission range. In the case where the controlamount of the control device has reached an upper limit beyond which theelectric construction of the electricity supplying device cannot respondto command signals from the control device to change, i.e., increase ordecrease the electricity supplied to the heater roller, or in the casewhere the amount or rate of decreasing of the electric resistance of theheater roller because of jamming of the recording sheet is much greaterthan the amount or rate of decreasing of the electricity supply underthe control of the control device, the control device may not beeffective in causing the electricity measured by the measuring device toremain within the control range and therefore the measured electricitymay eventually deviate from the permission range. In either case, thestopping means stops the supplying of the electricity from the supplyingdevice to the heater roller, thereby effectively preventing the heaterroller from generating an excessively great amount of thermal energy.

It is a second object of the present invention to provide a thermalimage-fixing apparatus capable of fixing an unfixed image on a recordingsheet with an appropriate amount of thermal energy.

The second object has been achieved by the present invention. Accordingto a second aspect of the present invention, there is provided animage-fixing apparatus comprising: a presser roller which is rotatableabout a first rotation axis thereof; a heater roller which is rotatableabout a second rotation axis thereof parallel to the first rotation axisand which is held in contact with the presser roller to provide a nipperfor nipping a recording sheet bearing an unfixed image thereon, anelectric resistance of the heater roller decreasing as respectivecontact areas of the heater and presser rollers which areas define thenipper increase; a supplying device which supplies an electricity to theheater roller so that the nipper generates a thermal energy to thermallyfix the unfixed image on the recording sheet; a measuring device whichmeasures the electricity supplied from the supplying device to theheater roller; a first sheet-data obtaining device which obtains firstsheet data representing at least one thermal parameter of the recordingsheet; a first memory in which is stored a first relationship betweenthe at least one thermal parameter of the recording sheet and a controlrange regarding the electricity; and a control device which controls thesupplying of the electricity from the supplying device to the heaterroller, so that the electricity measured by the measuring device fallswithin the control range corresponding to the at least one thermalparameter of the recording sheet represented by the first sheet dataobtained by the first sheet-data obtaining device.

In the thermal image-fixing apparatus in accordance with the secondaspect of the invention, the control device controls the supplying ofthe electricity from the supplying device to the heater roller, so thatthe electricity measured by the measuring device falls within thecontrol range corresponding to the at least one thermal parameter of therecording sheet represented by the first sheet data obtained by thefirst sheet-data obtaining device. For example, in the case where theelectric resistance of the heater roller changes because of use ofvarious recording sheets having, e.g., different thickness values, thecontrol device controls the electricity supplied to the heater roller soas to fall within the control range. Thus, the present image-fixingapparatus fixes an unfixed image on each recording sheet with anappropriate amount of thermal energy.

In a preferred embodiment in accordance with the second aspect of theinvention, the heater roller comprises: (a) a cylindrical resilientlayer which is formed of an electrically insulating and resilientmaterial, is concentric with the second rotation axis, and has aplurality of through holes formed through a thickness thereof; (b) afirst cylindrical electrode layer which is concentric with the secondrotation axis and has a plurality of electrodes each of which extendsinto a corresponding one of the through holes of the resilient layer;(c) a cylindrical resistant layer concentric with the second rotationaxis, the resistant layer and the first electrode layer sandwiching theresilient layer; and (d) a second cylindrical electrode layer concentricwith the second rotation axis, the second electrode layer and theresilient layer sandwiching the resistant layer, a portion of theresilient layer corresponding to the nipper being resiliently deformableso that a portion of the first electrode layer corresponding to thenipper is held in electric contact via the electrodes thereofcorresponding to the nipper with a portion of the resistant layercorresponding to the nipper, the supplying device supplying theelectricity to the heater roller through the first and second electrodelayers so that the portion of the resistant layer corresponding to thenipper generates the thermal energy to thermally fix the unfixed imageon the recording sheet.

In another embodiment in accordance with the second aspect of theinvention, the first sheet-data obtaining device comprises asheet-parameter measuring device which measures, as the first sheetdata, the at least one thermal parameter of the recording sheet. Thesheet-parameter measuring device may measure, as the at least onethermal parameter of the recording sheet, at least one of a width, athickness, a surface roughness, and a moisture content of the recordingsheet.

In yet another embodiment in accordance with the second aspect of theinvention, the image-fixing apparatus further comprises a secondsheet-data obtaining device which obtains second sheet data representingat least one of a width and a thickness of the recording sheet; and asecond memory in which is stored a second relationship between the atleast one of the width and the thickness of the recording sheet and apermission range regarding the electricity. The second memory may store,as the second relationship, a second data table including a plurality ofpermission ranges corresponding to a plurality of values of the at leastone of the width and the thickness of the recording sheet. The controldevice may comprise: second selecting means for selecting, from thesecond data table stored in the second memory, one of the permissionranges corresponding to the at least one of the width and the thicknessof the recording sheet represented by the second sheet data obtained bythe second sheet-data obtaining device; and stopping means for stoppingthe supplying of the electricity from the supplying device to the heaterroller, when the electricity measured by the measuring device does notfall within the selected one permission range. The selected onepermission range may fully cover the control range. In the last case,the difference of the respective upper limits of the selected controland permission ranges and the difference of the respective lower limitsof the selected two ranges may be pre-determined to define an upper anda lower limit of a "rate" permission range for the rate of change of theelectricity supplied to the heater roll, respectively. If the differenceof two values of the electricity measured by the measuring device duringtwo successive control cycles of the control device exceeds the upper orlower limit of the "rate" permission range, then the stopping meansstops the supplying of the electricity to the heater roller, because thejamming of the recording sheet has occurred at the nipper.

It is a third object of the present invention to provide a thermalimage-fixing apparatus capable of quickly detecting the occurrence ofjamming of a recording sheet bearing an unfixed image thereon.

The third object has been achieved by the present invention. Accordingto a third aspect of the present invention, there is provided animage-fixing apparatus comprising: a presser roller which is rotatableabout a first rotation axis thereof; a heater roller which is rotatableabout a second rotation axis thereof parallel to the first rotation axisand which is held in contact with the presser roller to provide a nipperfor nipping a recording sheet bearing an unfixed image thereon, anelectric resistance of the heater roller decreasing as respectivecontact areas of the heater and presser rollers which areas define thenipper increase; a supplying device which supplies an electricity to theheater roller so that the nipper generates a thermal energy to thermallyfix the unfixed image on the recording sheet; a measuring device whichmeasures the electricity supplied from the supplying device to theheater roller; and a sheet-jamming detecting device which detects ajamming of the recording sheet at the nipper, based on the electricitymeasured by the measuring device.

In the thermal image-fixing apparatus in accordance with the thirdaspect of the invention, the sheet-jamming detecting device detects ajamming of the recording sheet at the nipper, based on the electricitymeasured by the measuring device. When the jamming of the recordingsheet occurs at the nipper, the "effective" area of the nipper increasesand accordingly the electric resistance of the heater roller decreases,so that the electricity supplied to the heater roller may increase. Inthis case, the sheet-jamming detecting device detects a jamming of therecording sheet at the nipper, based on the increasing of theelectricity measured by the measuring device. The increasing of theelectricity may be either an amount or a rate of increasing of theelectricity.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and optional objects, features, and advantages of the presentinvention will be better understood by reading the following detaileddescription of the preferred embodiments of the invention whenconsidered in conjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a thermal image-fixing apparatusembodying the present invention, taken along line 1--1 in FIG. 2;

FIG. 2 is a perspective view of the image-fixing apparatus of FIG. 1;

FIG. 3 is an enlarged cross-sectional view of a heater roller of theimage-fixing apparatus of FIG. 1;

FIG. 4(A) is an enlarged perspective view of respective portions of aninner electrode layer and a resilient layer of the heater roller of FIG.3;

FIG. 4(B) is an enlarged perspective view of a portion of the resilientlayer of the heater roller of FIG. 3;

FIG. 4(C) is an enlarged perspective view of a portion of the innerelectrode layer of the heater roller of FIG. 3;

FIG. 5 is a perspective view of the heater roller of FIG. 3;

FIG. 6 is an enlarged cross-sectional view of a nipper, A, provided bythe heater roller and a presser roller of the image-fixing apparatus ofFIG. 1;

FIG. 7(A) is an enlarged cross-sectional view of a portion of the heaterroller of FIG. 3 taken along line 7A--7A in FIG. 6 and not correspondingto the nipper A;

FIG. 7(B) is an enlarged cross-sectional view of a portion of the heaterroller of FIG. 3 taken along line 7B--7B in FIG. 6 and corresponding tothe nipper A;

FIG. 8 is a diagrammatic view of the electric circuit of theimage-fixing apparatus of FIG. 1;

FIG. 9 is a flow chart according to which the image-fixing apparatus ofFIG. 1 is operated;

FIG. 10 is a flow chart according to which another image-fixingapparatus as a second embodiment of the invention is operated;

FIG. 11 is a flow chart corresponding to FIG. 10, according to whichanother image-fixing apparatus as a third embodiment of the invention isoperated;

FIG. 12 is a diagrammatic view of the electric circuit of anotherimage-fixing apparatus as a fourth embodiment of the invention; and

FIG. 13 is a flow chart corresponding to FIG. 10, according to which theimage-fixing apparatus of FIG. 12 is operated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIGS. 1 and 2, there is shown a thermal image-fixingapparatus 1 embodying the present invention. The image-fixing apparatus1 may be employed in a printer of an electrophotographic type, forexample.

The image-fixing apparatus 1 includes an upper and a lower covering 10,20 each of which has a generally U-shaped cross section as seen inFIG. 1. The upper and lower coverings 10, 20 cooperate with each otherto define, on one hand, a sheet inlet 11 through which a recording sheet50 bearing an unfixed toner image 51 thereon is fed in and, on the otherhand, a sheet outlet 12 through which the recording sheet 50 bearing thetoner image 51 which has thermally been fixed is fed out.

The image-fixing apparatus 1 also includes a heater roller 30 rotatablysupported by the wall of the upper covering 10, and a presser roller 40rotatably supported by the lower covering 20. The heater roller 30includes a first axis member 31 rotatably supported at opposite axialends 31a, 31b (FIG. 5) thereof via respective bearings (not shown) bythe wall of the upper covering 10.

The presser roller 40 includes a second axial member 41 rotatablysupported at opposite axial ends thereof via respective bearings (notshown) by the wall of the lower covering 20. The presser roller 40additionally includes a resilient roller body 42 provided on the outercircumferential surface of the second axis member 41. The roller body 42is formed of a heat-resistant, resilient material such as a siliconerubber. The roller body 42 of the presser roller 40 is held in contactwith the heater roller 30, so that a nipper, A, is provided for nippingthe recording sheet 50 being fed in through the sheet inlet 11. At thenipper A, the roller body 42 of the presser roller 40 is resilientlydeformable as shown in FIG. 1.

In the present image-fixing apparatus 1, the toner image 51 is fixed onthe recording sheet 50 by fusing the toner with a thermal energygenerated by the heater roller 30 in a manner described later. Forthermally fixing the toner image 51 on the recording sheet 50, theheater roller 30 and the presser roller 40 are rotated in respectivedirections indicated at arrows in FIG. 1 so as to feed the recordingsheet 50 forward in a direction indicated at an arrow, at an appropriatespeed. Thus, the present image-fixing apparatus 1 has two functions, oneis to feed the recording sheet 50 and the other is to thermally fuse thetoner and thereby fixing the toner image 51 on the sheet 50. The secondaxis member 41 of the presser roller 40 is connected via gears (notshown) to the first axis member 31 of the heater roller 30, so that therotation of the presser roller 40 is interlocked with the rotation ofthe heater roller 30. In FIG. 1, reference numerals 60a, 60brespectively designate an inlet-side sheet guide and an outlet-sidesheet guide each for guiding the recording sheet 50, and referencenumerals 70a, 70b designate a pair of sheet discharging rollers fordischarging the recording sheet 50 out of the present apparatus 1.

FIG. 2 shows the external appearance of the present image-fixingapparatus 1. The apparatus 1 further includes four reflection-typeoptical sensors 8a, 8b, 8c, 8d which cooperate with each other tomeasure the width of the recording sheet 50. The array of four opticalsensors 8a to 8d are supported by the upper covering 10, along the sheetinlet 11. In the present apparatus 1, the recording sheet 50 is fed invia the sheet inlet 11 such that a left-hand end 11a of the inlet 11 isused as a reference for the feeding of the sheet 50. For example, therecording sheet 50 is fed in while the left-hand edge of the sheet 50keeps a prescribed distance from the left-hand end 11a of the sheetinlet 11. The first sensor 80a detects a recording sheet 50 of a B5size; the first and second sensors 80a, 80b detect an A4-size sheet 50;the first to third sensors 80a to 80c detect a B4-size sheet 50; and thefirst to fourth sensors 80a to 80d detect an A3-size sheet 50. Each ofthe B5, A4, B4, and A3 sizes defines the prescribed length and width ofa corresponding cut sheet in accordance with Japanese IndustrialStandards (JIS). Reference numeral 90 designates a gear for transmittinga driving force to the first axis member 31 of the heater roller 30.

Hereinafter, there will be described in detail the construction of theheater roller 30 by reference to FIGS. 3 to 7. The heater roller 30 hasthe two functions, one is to feed the recording sheet 50 and the otheris to generate a thermal energy at the nipper A and thermally fuse thetoner and thereby fix the toner image 51 on the sheet 50. The first axismember 31 of the heater roller 30 is formed of a metallic material. Theheater roller 30 additionally includes a cylindrical base body 32 whichis provided on the outer circumferential surface of the first axismember 31 concentrically with the axis member 31. The base body 32 isformed of a more or less resilient, electrically insulating materialsuch as a rubber or a resin.

The heater roller 30 further includes a cylindrical inner electrodelayer 33 which is fit on the outer circumferential surface of the basebody 32 concentrically with the first axis member 31. The innerelectrode layer 33 is formed of an electrically conducting metallicmaterial such as an aluminum-based material. As shown in FIGS. 4(a) and4(c), the inner electrode layer 33 includes, on the outercircumferential surface thereof, a multiplicity of columnar electrodes33a which are arranged in prescribed arrays and spaced away from eachother. Upon application of an electricity to the inner electrode layer33, the electricity is conductable to the electrodes 33a via the innerelectrode layer 33. In the present embodiment, each electrode 33a isformed of a heat-resistant and wearing-resistant metallic material suchas tungsten, because the top end of each electrode 33a is subject tohigh temperatures and high pressures. Each electrode 33a may take adifferent shape than the columnar shape, such as a cubic orsemi-spherical shape. The electrodes 33a may otherwise be formed of thesame material as the material of the inner electrode layer 33, and mayalso be formed integrally with the same 33.

The heater roller 30 further includes a cylindrical resilient layer 34which is formed of an electrically insulating, resilient material andwhich is fit on the outer circumferential surface of the inner electrodelayer 33 concentrically with the first axis member 31 as shown in FIG.3. The resilient material may be selected from natural rubber (NR),isoprene rubber (IR), butadiene rubber (BR), styrene butadiene rubber(SBR), butyl rubber (IIR), ethylene propylene rubber (EPM, EPDM),silicone rubber (Si), chloroprene rubber (CR), hypalon (CSM), fluororubber (FPM), and urethane rubber (V). As shown in FIGS. 4(a) and 4(b),the resilient layer 34 has a plurality of cylindrical through holes 34aeach formed through the thickness thereof. As shown in FIG. 4(a), eachof the columnar electrodes 33a extends into a corresponding one of thecylindrical through holes 34a of the resilient layer 34 substantiallyconcentrically with the corresponding one through hole 34a. The depth ofeach through hole 34a is greater than the height of each electrode 33a.With the resilient layer 34 being assembled on the inner electrode layer33, the top end of each electrode 33a does not reach the level of theouter circumferential surface of the resilient layer 34, i.e., the levelof the top end of each through hole 34a. The diameter of each throughhole 34a is greater than the diameter of each electrode 33a.

The heater roller 30 further includes a cylindrical resistant layer 35which is formed of a material obtained by dispersing carbon particles ina polycarbonate film and which has a thickness of about 20 micrometers(μm) and a prescribed volume resistivity. As shown in FIG. 3, theresistant layer 35 is fit on the outer circumferential surface of theresilient layer 34 concentrically with the first axis member 31. Theresistant layer 35 and the inner electrode layer 33 sandwiches theresilient layer 34 located therebetween.

The heater roller 30 further includes a cylindrical outer electrodelayer 36 which is formed by vacuum deposition of a material such asaluminum and which has a thickness of about 1,000 angstroms to 0.2millimeters (mm) and which is fit on the outer circumferential surfaceof the resistant layer 35 concentrically with the first axis member 31.

The heater roller 30 additionally includes a cylindrical protectionlayer 37 which is formed of a material such as ethylene tetrafluorideand which is fit on the outer circumferential surface of the outerelectrode layer 36 concentrically with the first axis member 31. Theprotection layer 37 defines the outer circumferential surface of theheater roller 30, and prevents the toner on the recording sheet 50 frombeing adhered by fusing to the heater roller 30.

As shown in FIG. 5, respective right-hand ends 37a of the resilientlayer 34, resistant layer 35, outer electrode layer 36, and protectionlayer 37 are short, so that a right-hand end 33b of the inner electrodelayer 33 is exposed. A free end of a positive electrode 38a is held inelectric contact with the right-hand end 33b of the inner electrodelayer 33. Meanwhile, a left-hand end 37b of the protection layer 37 isshort, so that a left-hand end 36a of the outer electrode layer 36 isexposed. A free end of a negative electrode 38b is held in electriccontact with the left-hand end 36a of the outer electrode layer 36. Thepositive and negative electrodes 38a, 38b are fixed at respective baseportions thereof to the wall of the upper covering 10.

As shown in FIG. 6, the presser roller 40 is held in contact with theheater roller 30 so as to provide the nipper A for nipping the recordingsheet 50 on which the unfixed toner image 51 has been formed. A portionof the resilient layer 34 of the heater roller 30 corresponding to thenipper A is resiliently deformable to become thinner so that a portionof the inner electrode layer 33 of the heater roller 30 corresponding tothe nipper A is held in electric contact via the electrodes 33acorresponding to the nipper A with a portion of the resistant layer 35of the heater roller 30 corresponding to the nipper A. Similarly, theresilient roller body 42 of the presser roller 40 is resilientlydeformable because of the contact with the heater roller 30, and becomesthinner at the nipper A. Thus, the nipper A provides a sufficientpressing area and force for thermally fixing the toner image 51 on therecording sheet 50.

While the "nipper" portion of the resilient layer 34 currentlycorresponding to the nipper A is resiliently deformed, the remainingportion of the resilient layer 34 other than the "nipper" portionthereof is not deformed. Therefore, the electrodes 33a of the innerelectrode layer 33 corresponding to the remaining, undeformed portion ofthe resilient layer 34 are kept apart from the resistant layer 35, asshown in FIG. 7(A). Thus, a direct current cannot flow through theresistant layer 35. On the other hand, at the nipper A where the heaterand presser rollers 30, 40 are held in contact with each other, theresilient layer 34 is deformed and made thinner, so that the electrodes33a of the inner electrode layer 33 are brought into electric contactwith the resistant layer 35, as shown in FIG. 7(B), and so that anelectricity is conductable through the resistant layer 35. The elementsand layers 31, 32, 33, 34, 35, 36, 37 of the heater roller 30 areuniformly adhered at respective interfaces thereof to each other with asuitable adhesive force produced by an adhesive or a vapor deposition.

Next, there will be described the electric circuit of the presentimage-fixing apparatus 1 by reference to FIG. 8.

A sensor circuit 150 receives respective detection signals from the fouroptical sensors 80a, 80b, 80c, 80d, and supplies a sheet-size signalrepresenting the detected size of the recording sheet 50, to a controldevice 120 which is constituted by a microcomputer. A table memory 140such as a read only memory (ROM) stores a table representing arelationship between (A) the B5, A4, B4, and A3 sizes of recordingsheets 50 and (B) (b1) corresponding center electric current values, Ia,and (b2) corresponding permission half-width values, C. The controldevice 120 selects one of the center values Ia and one of the permissionhalf-width values C which corresponds to the detected size of therecording sheet 50 represented by the sheet-size signal from the sensorcircuit 150.

A heater-roller drive circuit 130 generates a drive signal and a stopsignal to rotate or stop the heater roller 30, according to respectivecontrol signals supplied from the control device 120. A display device160 displays a "jamming" indication, as described later, when a sheetjamming has occurred at the nipper A.

An electric-current measuring circuit 100 measures the magnitude of adirect electric current flowing into the exposed end 33b of the innerelectrode layer 33 of the heater roller 30 via the positive electrode38a from an electric power source circuit 110, and generates anelectric-current signal representing the detected magnitude of theelectric current. The positive terminal of the power source circuit 110is connected to the positive electrode 38a via the electric-currentmeasuring circuit 100, and the negative terminal of the power sourcecircuit 110 is connected to the negative electrode 38b. The power sourcecircuit 110 adjusts, under the control of the control device 120, themagnitude of the electric current flowing between the exposed end 33b ofthe inner electrode layer 33 and the the exposed end 36a of the outerelectrode layer 36 via the electric-current measuring circuit 100 andthe positive and negative electrodes 38a, 38b.

The control device 120 cooperates with the sensor circuit 150 and theelectric-current measuring circuit 100 to execute a control programrepresented by the flow chart of FIG. 9. Thus, the control device 120supplies control signals to the power source circuit 110 and theheater-roller drive circuit 130. The above-indicated control program ispre-stored in a read only memory (ROM, not shown) of the control device120.

Hereinafter, there will be described the operation of the presentapparatus 1 constructed as described above for thermally fixing theunfixed toner image 51 on the recording sheet 50. Once the presentapparatus 1 is placed in an operative state, the control device 120starts executing the control program represented by the flow chart ofFIG. 9. It is assumed that, at this point of time, the recording sheet50 on which the unfixed toner image 51 has been formed is being nippedby the nipper A provided by the cooperation of the heater roller 30 andthe presser roller 40, as shown in FIG. 1. As described above, at thenipper A, some electrodes 33a of the inner electrode layer 33 are heldin contact with the resistant layer 35 via the deformed portion of theresilient layer 34 (FIG. 7(B)). It is also assumed that at this point oftime no electric voltage is being applied to the heater roller 30.

First, at Step 11, a central processing unit (CPU, not shown) of thecontrol device 120 supplies a drive command signal to the heater-rollerdrive circuit 130 to rotate the heater roller 30, and an applicationcommand signal to the power source circuit 110 to apply a directelectric current to the heater roller 30. Thus, the heater roller 30 isrotated together with the presser roller 40 by being driven by the drivecircuit 130, and simultaneously the power source circuit 110 supplies adirect electric current to a portion of the heater roller 30corresponding to the nipper A via the positive and negative electrodes38a, 38b and the exposed ends 33b, 36a of the inner and outer electrodelayers 33, 36. Consequently the recording sheet 50 is fed by therotations of the heater and presser rollers 30, 40 while being nipped bythe nipper A, and simultaneously the direct current flows through aclosed circuit constituted by the power source circuit 110, theelectric-current measuring circuit 100, the positive electrode 38a, theinner electrode layer 33, and the electrodes 33a corresponding to thenipper A, the resistant layer 35, the outer electrode layer 36, and thenegative electrode 38b. Thus, at only the nipper portion of the heaterroller 30, the resistant layer 35 generates an amount of heatcorresponding to the magnitude of resistivity thereof as a result of theflowing of the direct current therethrough. The thermal energycorresponding to the generated heat is conducted from the resistantlayer 55 to the recording sheet 50 via respective portions of the outerelectrode layer 36 and the protection layer 37 corresponding to thenipper A. The conducted thermal energy fuses the toner on the recordingsheet 50, thereby fixing the toner image 51 on the sheet 50, at thenipper A. The thermal fixing of the toner image 51 on the recordingsheet 50 is carried out while at the same time the sheet 50 is fedforward by the rotations of the heater and presser rollers 30, 40.

Since each of the outer electrode layer 36 and the protection layer 37is very thin, the thermal resistance of each layer 36, 37 is very low.Therefore, the thermal energy generated at the resistant layer 35 isconducted with high efficiency to the outer surface of the heater roller30. Thus, the heater roller 30 is placed very quickly in afixing-starting state where the heater roller 30 can start the thermalfixation of the toner image 51 on the recording sheet 50. That is, thewaiting or building-up time of the heater roller 30 needed to start thethermal image fixation is minimized. Since the heat generation andconduction of the heater roller 30 is limited to the nipper A, the otherelements of the present apparatus 1 are effectively prevented from beingadversely affected by the heat generated by the heater roller 30.Additionally, the electric consumption of the present apparatus 1 issignificantly reduced. Since the number of heat-insulating membersemployed in the present apparatus 1 can be decreased, the apparatus 1can be produced in a smaller size.

Step S11 is followed by Step S12 where the control device 120 or the CPUthereof identifies the size of the recording sheet 50 based on thesheet-size signal supplied from the sensor circuit 150. It is assumedthat the CPU identifies the B4 size of the recording sheet 50.Subsequently, at Step S13, the CPU selects, from the table memory 140, acenter current value Ia and a permission half-width value C eachcorresponding to the B4-size recording sheet 50. Step S13 is followed byStep S14 to set the selected values Ia, C as control values used tocontrol the electric current supplied to the heater roller 30.Subsequently, at Step S15, the CPU reads an instant magnitude, I, of theelectric current flowing through the heater roller 30, based on theelectric-current signal supplied from the electric-current measuringcircuit 100. Step S15 is followed by Step S16 to judge whether themeasured magnitude I of the electric current is greater than an upperlimit value obtained by adding the permission half-width value C to thecenter current value Ia. If a negative judgment is made at Step S16,that is, if the measured value I is not greater than the upper limitIa+C, the control of the CPU goes to Step S17. At Step S17, the CPUjudges whether the measured magnitude I is smaller than a lower limitvalue obtained by subtracting the permission half-width value C from thecenter current value Ia. If a negative judgment is made at Step S17,that is, if the measured value I is greater than the lower limit Ia-C,the control of the CPU goes to Step S18.

At Step S18, the CPU judges whether an image-fixing operation has beencompleted on the recording sheet 50. To this end, a sheet sensor may beemployed, which includes a small lever and a photointerrupter. The sheetsensor detects the leading end of the recording sheet 50 when the smalllever is first pivoted to interrupt the photointerrupter by beingbrought into engagement with the recording sheet 50, and detects thetrailing end of the sheet 50 when the small lever is again pivoted tointerrupt the photointerrupter by releasing from the engagement with thesheet 50. If a negative judgment is made at Step S18, that is, if theimage-fixing operation has not been completed, the control of the CPUgoes back to Step S15 and the following steps so as to continue theimage-fixing operation while monitoring the electric current I flowingthrough the heater roller 30. Meanwhile, if the image-fixing operationis ended, a positive judgment is made at Step S18, and the currentcontrol cycle is ended.

On the other hand, for example, if the recording sheet 50 is jammed atthe heater roller 30 and consequently the effective area of the nipper Aincreases, the number of the electrodes 33a brought into electriccontact with the resistant layer 35 excessively increases, and theoverall electric resistance of the heater roller 30 decreases.Accordingly an excessively great electric current flows through theheater roller 30. In this case, the measured current I exceeds the upperlimit Ia+C, and a positive judgment is made at Step S16, so that thecontrol of the CPU of the control device 120 goes to Step S19. At StepS19, the CPU supplies a stop command signal to the heater-roller drivecircuit 130 to stop the rotation of the heater roller 30, and at StepS20 the CPU supplies a shut-off command signal to the power sourcecircuit 110 to shut off the supplying of direct-current voltage to theheater roller 30. Consequently the heater-roller drive circuit 130 stopsthe rotations of the heater and presser rollers 30, 40, and the powersource circuit 110 stops the supplying of direct-current voltage to theheater roller 30. Thus, the various elements of the present apparatus 1are effectively prevented from being deteriorated or even broken byabnormal heat generation or abnormal high temperatures. Step S20 isfollowed by Step S21 where the control device 120 or the CPU supplies anindication signal to the display device 160 so that the display device160 displays a "disorder" indication that a disorder (e.g., jamming ofthe recording sheet 50) has occurred and that the supplying ofdirect-current voltage to the heater roller 30 has been stopped.Meanwhile, for example, if a certain disorder occurs to the presentapparatus 1 and no electric current can flow through the heater roller30, the present apparatus 1 cannot fix the toner image 51 on therecording sheet 50. In this case, the measured current I becomes smallerthan the lower limit Ia-C and a positive judgment is made at Step S17,so that the control of the CPU goes to Step S19 and the following stepsas described above.

It emerges from the foregoing description that, in the presentembodiment, the table memory 140 stores the data table representing therelationship between the various sizes of recording sheets 50 and thecorresponding center current values Ia and corresponding permissionhalf-width values C, the four optical sensors 80a-80d cooperate witheach other to identify the size of each recording sheet 50, and thecontrol device 120 selects the center current value Ia and permissionhalf-width value C corresponding to the identified size of the recordingsheet 50. If the measured current I exceeds the upper limit Ia+C, thecontrol device 120 stops the rotations of the heater and presser rollers30, 40 and stops the supplying of electric current to the heater roller30, thereby stopping the image-fixing operation being effected on therecording sheet 50.

In the present embodiment, the table memory 140 stores lower centercurrent values Ia and smaller permission half-width values C fornarrower recording sheets 50. Therefore, for example, in the case wherea narrower recording sheet 50 is jammed and a lower electric currentflows than where a wider recording sheet 50 is jammed, the presentapparatus 1 can accurately detect the jamming of the narrower sheet 50and quickly stops the image-fixing operation being effected on the sheet50. On the other hand, in the case where a higher electric current ismeasured while an image-fixing operation is effected on a widerrecording sheet 50, the present apparatus 1 cannot erroneouslyidentifies that the wider sheet 50 is being jammed.

Additionally, the table memory 140 stores different permissionhalf-width values C for different sheet sizes. Each of the half-widthvalues C may be given a special considerations practically needed for acorresponding sheet size. For example, a recording sheet 50 identifiedas having a post-card size would probably be a post card, and thethickness of a post card is greater than that of a common cut sheethaving a comparable size. In the case where a toner image 51 is fixed ona post card 50, the effective area of the nipper A increases because ofthe greater thickness of the post card 50 and accordingly a greatercurrent flows than expected from the post-card size of the recordingsheet 50. Therefore, in the present embodiment, the table memory 140provides a greater permission half-width value C for the post-card size,for avoiding an erroneous identification that a post card-size recordingsheet 50 (this would probably be a post card) is being jammed. On theother hand, a recording sheet 50 having the A4 size would probably be acommon cut sheet, therefore the table memory 140 provides a smallerpermission half-width value C for the A4 size, for more quickly findingthe jamming of the A4-size recording sheet 50 (this would probably be acommon cut sheet).

Next, there will be described a second embodiment of the presentinvention by reference to FIG. 10. The second embodiment also relates toa thermal image-fixing apparatus and has the same hardware constructionas that shown in FIGS. 1-8, except that a table memory 140 stores a datatable representing a relationship between (A) the B5, A4, B4, and A3sizes of recording sheets 50 and (B) (b1) the corresponding centerelectric current values Ia, (b2) the corresponding permission half-widthvalues C, and additionally (b3) corresponding control half-width values,D. The same reference numerals as used in FIGS. 1-8 for the firstembodiment are used to designate the corresponding elements of thesecond embodiment. A control device 120 of the image-fixing apparatus inaccordance with the second embodiment carries out a control programrepresented by the flow chart of FIG. 10, in place of the controlprogram represented by the flow chart of FIG. 9 for the firstembodiment.

First, the control device 120 or the CPU thereof carries out Step S51that is the same as Step S11 of FIG. 9. Thus, the present apparatus 1starts the thermal fixing of an unfixed toner image 51 on a recordingsheet 50.

Step S52 is the same as Step S12 of FIG. 9. Step S53 is almost the sameas Step S13 of FIG. 9, and the former is different from the latter inthat the CPU selects, from the table memory 140, a control half-widthvalue D in addition to the center current value Ia and the permissionhalf-width value C, each corresponding to the size (e.g., B4) of therecording sheet 50 identified at Step S52. If other sheet sizes areidentified, the CPU selects, from the table memory 140, the appropriatevalues Ia, C, D corresponding to each identified sheet size. At thefollowing Step S54, the CPU sets the selected values Ia, C, D as controlvalues used to control the electric current supplied to a heater roll30. Step S55 is the same as Step S15 of FIG. 9. Each of the controlhalf-width values D is a positive value smaller than a correspondingpositive permission half-width value C.

Since Steps S56 and S57 are the same as Steps S16 and S17 of FIG. 9,respectively, the repeated description of those steps is omitted. If anelectric current I measured at Step S55 falls within a permission rangehaving an upper limit value, Ia+C, obtained by adding the permissionhalf-width value C to the center current value Ia and a lower limitvalue, Ia-C, obtained by subtracting the permission half-width value Cfrom the center current value Ia, there is no need to adjust theelectric current I. In this case, a negative judgment is made at each ofSteps S56 and S57, and the control of the CPU goes to Step S58, becausea control range from Ia-D to Ia+D entirely falls within the permissionrange from Ia-C to Ia+C. At Step S58, the CPU judges whether themeasured current I is excessively great and should be adjusted, i.e.,the electric current I is greater than the upper limit value Ia+Dobtained by adding the control half-width value D to the center currentvalue Ia. If a negative judgment is made at Step S58, the control of theCPU goes to Step S59. At Step S59, the CPU judges whether the measuredcurrent I is excessively small and should be adjusted, i.e., theelectric current I is smaller than the lower limit value Ia-D obtainedby subtracting the control half-width value D from the center currentvalue Ia. In the case where the electric current I falls within thecontrol range of Ia±D, a negative judgment is made at each of Steps S58and S59, and the control of the CPU goes to Step S60 that is the same asStep S18 of FIG. 9. Thus, if the judgments made at Steps S56 to S59identify that the measured current I falls within the permission rangeof Ia±C and simultaneously within the control range of Ia±D, the CPUdoes not adjust the electric current I and the control thereof proceedswith Step S60. If a negative judgment is made at Step S60, that is, ifan image-fixing operation has not been completed on the recording sheet50, the control of the CPU goes back to Step S55 and the followingsteps. Meanwhile, if a positive judgment is made at Step S60, that is,if the image-fixing operation has been completed on the sheet 50, thecurrent control cycle according to the flow chart of FIG. 10 is ended.

The measured current I does not fall within the permission range of Ia±Cif, for example, the effective area of the nipper A defined byrespective contact or deformed areas of the heater and presser rollers30, 40 largely increases because of the jamming of the recording sheet50 and an excessively great current flows through the heater roller 30.In this case, the electric current I exceeds the upper limit value Ia+Cof the permission range of Ia±C, and a positive judgment is made at StepS56. Subsequently, the control of the CPU goes to Steps S63, S64 and S65that are the same as Steps S19, S20, and S21 of FIG. 9. Also, themeasured current I does not fall within the permission range of Ia±C if,for example, no electric current is supplied to the heater roller 30because of some disorder of the present apparatus 1. In the last case,the electric current I is smaller than the lower limit value Ia-C of thepermission range of Ia±C, and a positive judgment is made at Step S57.Then, the control of the CPU goes to Steps S63, S64 and S65.

In the case where the measured current I falls within the permissionrange of Ia±C but does not fall within the control range of Ia±D, forexample, in the event that the measured current I is smaller than theupper limit Ia+C of the permission range but is greater than the upperlimit Ia+D of the control range while an image-fixing operation iscarried out on a thick recording sheet 50, a positive judgment is madeat Step S58 and the control of the CPU goes to Step S61. At Step S61,the CPU supplies a control signal to the power source circuit 110 so asto decrease the electric current I by a predetermined small unit amountor decrement and thereby adjust the current I. Then, the control of theCPU returns to Step S55 and the following steps. On the other hand, inthe event that the measured current I is greater than the lower limitIa-C of the permission range but is smaller than the lower limit Ia-D ofthe control range while an image-fixing operation is carried out on athin recording sheet 50, a positive judgment is made at Step S59 and thecontrol of the CPU goes to Step S62. At Step S62, the CPU supplies acontrol signal to the power source circuit 110 so as to increase theelectric current I by a predetermined small unit amount or increment andthereby adjust the current I. Then, the control of the CPU returns toStep S55 and the following steps.

In the second embodiment described above by reference to the flow chartof FIG. 10, the control device 120 or the CPU controls or adjusts theelectric current I so as to keep it within the control range of Ia±D,even if the effective area of the nipper A changes depending on therecording sheets 50 of different thickness values. Consequently tonerimages 51 are fixed with appropriate amounts of thermal energy on thethickness-different recording sheets 50. In addition, even if recordingsheets 50 of different width values are used, the control device 120adjusts the electric current I based on the control values (i.e., centercurrent value Ia, permission half-width value C, control half-widthvalue D) appropriate for the width (i.e., size) of each recording sheet50. Thus, an excellent fixation of the toner image 51 is effected oneach recording sheet 50.

In the first and second embodiments in accordance with the presentinvention, the thermal energy produced at a portion of the resistantlayer 35 corresponding to the nipper A is conducted to the recordingsheet 50 via respective portions of the outer electrode layer 36 and theprotection layer 37 each corresponding to the nipper A. Thus, at onlythe nipper A, the thermal energy conducted to the outer surface of theheater roller 30 is used to thermally fuse the toner image 51 on therecording sheet 50. The thermal fixation of the toner image 51 on thesheet 50 is effected with high quality while the sheet 50 is fed forwardat high speed.

Since the thermal energy generated in the "nipper" portion of theresistant layer 35 is conducted to the recording sheet 50 via the thin"nipper" portions of the outer layers 36, 37, the conduction of thethermal energy enjoys high efficiency. Therefore, the heater roller 30is very quickly placed in an image-fixing state in which the roller 30can start an image-fixing operation on a recording sheet 50. Thus, thewaiting or building-up time needed to start the image-fixing operationis minimized. Since the generation and conduction of the thermal energyare limited to respective portions of the heater and presser rollers 30,40 corresponding to the nipper A, the other elements of the image-fixingapparatus 1 are effectively prevented from adversely being affected bythe heat generated from the heater roller 30. In addition, the amount ofelectric power consumed by the present apparatus 1 is largely reduced.Moreover, the present image-fixing apparatus 1 is provided as a compactand reliable one.

Furthermore, since the image-fixing apparatus 1 has different permissionranges for different width values of recording sheets 50, the apparatus1 quickly detects the jamming of any recording sheet 50, and stops thesupplying of electric power to the heater roller 30, thereby preventingthe apparatus 1 from being broken because of excessively raisedtemperatures. Even if the effective area of the nipper A changesdepending upon the different thickness values of recording sheets 50 ofa same width, the apparatus 1 automatically adjusts the suppliedelectric power so as to maintain it within the control range appropriatefor that width. Consequently the toner images 51 are fixed on therecording sheets 50 of different thickness values with appropriateamounts of thermal energy. Moreover, even if recording sheets 50 ofdifferent width values are used, the apparatus 1 adjusts the suppliedelectric power to maintain it within the control range appropriate foreach of the different width values. Thus, the thermal fixation of thetoner image 51 on each recording sheet 50 is effected with high quality.

Referring further to FIG. 11, there is shown a flow chart representing acontrol program employed by a third embodiment of the present invention.The third embodiment has the same hardware construction as that of thefirst embodiment shown in FIGS. 1-8, except that a table memory 140stores a data table representing a relationship between (A) the B5, A4,B4, and A3 sizes of recording sheets 50 and (B) (b1) the correspondingcenter current values Ia and (b2) the corresponding control half-widthsD. In the flow chart of FIG. 11, the same steps as the steps employed inthe flow chart of FIG. 10 are given the same reference numerals and thedescription of those steps is omitted.

At Step S200 following Step S53, a CPU of a 5 control circuit 120 of thepresent image-fixing apparatus sets the center current value Ia and thecontrol half-width value D each corresponding to the size of a recordingsheet 50 identified at Step S52, as control values with which the CPUcontrols a power source circuit 110. After the CPU reads in an electriccurrent I supplied from the power source circuit 110 to a heater roller30, through an electric-current measuring circuit 100, at Step S55, thecontrol of the CPU goes to Step S201 to judge whether a content, T, of acounter falls within a "counter" permission range from -Ta to Ta(Ta >0). If a negative judgment is made at Step S201, the control goesto Steps S63 and the following steps.

On the other hand, if a positive judgment is made at Step S201, thecontrol of the CPU goes to Step S58. If a positive judgment is made atStep S58, the control goes to Step S61 to decrement the electric currentI by a small unit amount, and subsequently to Step S202 to subtract onefrom the content T of the counter. Then, the control of the CPU goesback to Step S55 and the following steps. Meanwhile, if a negativejudgment is made at Step S58, the control goes to Step S59. If apositive judgment is made at Step S59, the control goes to Step S62 toincrement the electric current I by a small unit amount, andsubsequently to Step S203 to add one to the content T of the counter.Then, the control of the CPU goes back to Step S55 and the followingsteps. Meanwhile, if a negative judgment is made at Step S59, thecontrol goes to Step S60, and subsequently to Step S204 to reset thecontent T of the counter to zero (T=0).

The content T of the counter represents the difference of the increasedand decreased amounts of the electric current I during the image-fixingoperation on the recording sheet 50. If the content T exceeds the upperlimit Ta of the "counter" permission range, it indicates the occurrenceof jamming of the recording sheet 50 at a nipper A provided by theheater roller 30 and a presser roller 40. The upper limit Ta ispre-determined such that the content T of the counter does not exceedthe upper limit Ta so long as the recording sheet 50 is fed forwardnormally, i.e., without jamming. The current content T of the counterrepresents an electric current which is taken as a result of carryingout of Steps S61 and S62, and the initial value T=0 of the counterrepresents an electric current which would otherwise be taken withoutcarrying out of Steps S61 and S62.

Referring further to FIGS. 12 and 13, there is shown a fourth embodimentof the invention also relating to a thermal image-fixing apparatus. Thepresent apparatus is different from the apparatus shown in FIGS. 1-8, inthat the former additionally has an electric-voltage measuring circuit200, and has a table memory 202 in place of the table memory 140 of thelatter, and a keyboard 204 in place of the optical sensors 80a to 80dand sensor circuit 150 of the latter. The electric-voltage measuringcircuit 200 measures an electric voltage of an electric power orelectricity supplied to a heater roller 30. The table memory 202 storesa data table representing a relationship between (A) the B5, A4, B4 andA3 sizes of recording sheets 50 and (B) (b1) the corresponding centercurrent values Ia and (b2) corresponding control current half-widths D,and additionally (b3) the corresponding center voltage values, Va, and(b4) corresponding permission voltage half-widths, E. The keyboard 204is operable by a user for inputting sheet data representing the B5, A4,B4, or A3 size of the recording sheet 50. In the flow chart of FIG. 13,the same steps as the steps used in the flow chart of FIG. 10 are giventhe same reference numerals and the description of those steps isomitted.

At Step S300 following Step S51, a CPU of a control circuit 120 of thepresent image-fixing apparatus reads in the sheet data input through thekeyboard 204 and stored in a RAM of the control circuit 120. Step S300is followed by Step S301 to select the center current value Ia, controlcurrent half-width value D, center voltage value Va, and permissionvoltage half-width value E each corresponding to the size of therecording sheet 50 specified at Step S300, as control values with whichthe CPU controls a power source circuit 110. After the CPU reads in anelectric current I and an electric voltage V supplied to a heater roller30, through an electric-current measuring circuit 100 and theelectric-voltage measuring circuit 200, at Step S303, the control of theCPU goes to Step S304 to judge whether the measured electric voltage Vis greater than an upper limit value obtained by adding the permissionvoltage half-width value E to the center voltage value Va. If a negativejudgment is made at Step S304, that is, if the measured voltage V is notgreater than the upper limit Va+E, the control of the CPU goes to StepS305. At Step S305, the CPU judges whether the measured voltage V issmaller than a lower limit value obtained by subtracting the permissionvoltage half-width value E from the center voltage value Va. If anegative judgment is made at Step S305, that is, if the measured voltageV is greater than the lower limit Va-E, the control of the CPU goes toStep S58. On the other hand, if a positive judgment is made at Step S304or S305, the control goes to Steps S63 and the following steps.

If a positive judgment is made at Step S58, the control goes to StepS306 to decrement the electric voltage V by a small unit amount. Then,the control of the CPU goes back to Step S303 and the following steps.Meanwhile, if a negative judgment is made at Step S58, the control goesto Step S59. If a positive judgment is made at Step S59, the controlgoes to Step S307 to increment the electric voltage V by a small unitamount. Then, the control of the CPU goes back to Step S303 and thefollowing steps. Meanwhile, if a negative judgment is made at Step S59,the control goes to Step S60.

The measured voltage V contains the control amounts increased and/ordecreased from the initial center voltage Va to maintain the electriccurrent I within the control range of Ia±D during the image-fixingoperation effected on the recording sheet 50. If the measured voltage Vexceeds the upper limit Va+E of the permission voltage range, itindicates the occurrence of jamming of the recording sheet 50 at anipper A provided by the heater roller 30 and a presser roller 40. Thecenter value Va and permission half-width value E are pre-determinedsuch that the measured voltage V does not exceed the upper limit Va+E solong as the recording sheet 50 is fed forward normally without jamming.The measured voltage V represents an electric voltage which is taken asa result of carrying out of Steps S306 and S307, and the initial centervoltage Va represents an electric current which would otherwise be takenwithout carrying out of Steps S306 and S307.

While the present invention has been described in its preferredembodiments, the present invention may otherwise be embodied.

For example, while in the illustrated embodiments the width of therecording sheet 50 is measured by the sheet-width sensors 80a to 80d, itis possible to employ other types of sensors such as a sensor whichidentifies different sizes of recording-sheet cartridges each adapted tobe inserted in a copying machine.

Additionally, although in the second and third embodiments the amount ofelectricity supplied to the heater roller 30 is monitored by measuringthe electric current I and is controlled by adjusting the electriccurrent I, it is possible to monitor an electric voltage supplied to theheater roller 30 and adjusts the electric voltage based on a permissionvoltage range and/or a control voltage range.

While in the first to third embodiments the optical sensors 80a to 80dare used to measure the width of the recording sheet 50, it is possibleto employ a sensor which measures the thickness of the recording sheet50, a sensor which measures the surface roughness of the recording sheet50, and/or a sensor which measures the moisture content of the recordingsheet 50, in place of or in addition to the sheet-width sensors 80a-80d.The sheet-thickness sensor may be of a type which includes an actuatoradapted to engage the upper surface of the sheet 50 and displacedepending upon the level of the upper surface of the same 50, and anamplifier enlarging the displacement of the actuator. Thesheet-roughness sensor is located in the path of feeding of therecording sheet 50. Since the air occurring in a rough surface of thesheet 50 functions as a heat insulator, the rough sheet 50 suffers lowthermal conductivity. Thus, the surface roughness of the sheet 50influences the amount of thermal energy needed to thermally fix thetoner image 51 on the sheet 50. The sheet-moisture sensor may be of atype which includes two electrodes for measuring an electricconductivity of the recording sheet 50. Since the electric conductivityhas high correlation with the moisture content, the moisture content ofthe sheet 50 may be obtained by measuring the electric conductivity ofthe sheet 50. Those electrodes may be embedded in a sheet feedingroller. When a more amount of thermal energy is applied to the sheet 50having a high water content, the degree of curling of the output sheetincreases. In these cases, the table memory 140, 202 may store a datatable representing a relationship between the different values ofthickness, surface roughness, or moisture content of recording sheets 50and the corresponding values Ia, C, and/or D.

In the illustrated embodiments, the table memory 140, 202 may bemodified to store a mathematical function defining a relationshipbetween the values of widths (sizes), thickness, surface roughness, ormoisture content of recording sheets 50 and the corresponding values Ia,Va, C, D, and/or E.

In the fourth embodiment, the keyboard 204 may be modified to inputsheet dimension or parameter data representing the thickness, surfaceroughness, or moisture content of the recording sheet 50, in place of orin addition to the width (size) of the same 50.

In the fourth embodiment, Steps S58 and S59 may be replaced by a step inwhich the CPU judges whether a measured electric power or electricity,W, is greater than the upper limit of a "electricity" permission rangeof Wa±F, and a subsequent step in which the CPU judges whether themeasured electricity W is smaller than the lower limit of the permissionrange of Wa±F. The measured electricity W is obtained as the product ofthe measured current value I and measured voltage value V.

It is to be understood that the present invention may be embodied withother changes, improvements, and modifications that may occur to thoseskilled in the art without departing from the spirit and scope of theinvention defined in the appended claims.

What is claimed is:
 1. An image-fixing apparatus comprising:a presserroller which is rotatable about a first rotation axis thereof; a heaterroller which is rotatable about a second rotation axis thereof parallelto said first rotation axis and which is held in contact with saidpresser roller to provide a nipper for nipping a recording sheet bearingan unfixed image thereon, an electric resistance of said heater rollerdecreasing as respective contact areas of said heater and presserrollers which areas define said nipper increase because of elasticdeformation of the heater roller; a supplying device which supplies anelectricity to said heater roller so that said nipper generates athermal energy to thermally fix said unfixed image on said recordingsheet; a measuring device which measures said electricity supplied fromsaid supplying device to said heater roller; a sheet-data obtainingdevice which obtains sheet data representing at least one of a width anda thickness of said recording sheet; a memory in which is stored arelationship between said at least one of said width and said thicknessof said recording sheet and a permission range regarding saidelectricity; and a control device which stops the supplying of saidelectricity from said supplying device to said heater roller, when saidelectricity measured by said measuring device does not fall within saidpermission range corresponding to said at least one of said width andsaid thickness of said recording sheet represented by said sheet dataobtained by said sheet-data obtaining device.
 2. An image-fixingapparatus according to claim 1, wherein said heater roller comprises:(a)a cylindrical resilient layer which is formed of an electricallyinsulating and resilient material, is concentric with said secondrotation axis, and has a plurality of through holes formed through athickness thereof; (b) a first cylindrical electrode layer which isconcentric with said second rotation axis and has a plurality ofelectrodes each of which extends into a corresponding one of saidthrough holes of said resilient layer; (c) a cylindrical resistant layerconcentric with said second rotation axis, said resistant layer and saidfirst electrode layer sandwiching said resilient layer; and (d) a secondcylindrical electrode layer concentric with said second rotation axis,said second electrode layer and said resilient layer sandwiching saidresistant layer,a portion of said resilient layer corresponding to saidnipper being resiliently deformable so that a portion of said firstelectrode layer corresponding to the nipper is held in electric contactvia the electrodes thereof corresponding to the nipper with a portion ofsaid resistant layer corresponding to the nipper, said supplying devicesupplying said electricity to said heater roller through said first andsecond electrode layers so that said portion of said resistant layercorresponding to said nipper generates said thermal energy to thermallyfix said unfixed image on said recording sheet.
 3. An image-fixingapparatus according to claim 1, wherein said sheet-data obtaining devicecomprises a sheet-dimension measuring device which measures, as saidsheet data, said at least one of said width and said thickness of saidrecording sheet.
 4. An image-fixing apparatus according to claim 1,wherein said sheet-data obtaining device comprises an input device whichis operable for inputting said sheet data representing said at least oneof said width and said thickness of said recording sheet.
 5. Animage-fixing apparatus according to claim 1, wherein said memory stores,as said relationship, a data table including a plurality of permissionranges corresponding to a plurality of values of said at least one ofsaid width and said thickness of said recording sheet.
 6. Animage-fixing apparatus according to claim 5, wherein said control devicecomprises:selecting means for selecting, from said data table stored insaid memory, one of said permission ranges corresponding to said atleast one of said width and said thickness of said recording sheetrepresented by said sheet data obtained by said sheet-data obtainingdevice; and stopping means for stopping said supplying of saidelectricity from said supplying device to said heater roller, when saidelectricity measured by said measuring device does not fall within theselected one permission range.
 7. An image-fixing apparatus according toclaim 1, further comprising an informing device which informs that saidcontrol device has stopped said supplying of said electricity from saidsupplying device to said heater roller.
 8. An image-fixing apparatusaccording to claim 1, further comprising a driving device which rotatesat least one of said heater and presser rollers to feed said recordingsheet, said control device comprising means for controlling said drivingdevice to stop the rotation of said at least one of said heater andpresser rollers and thereby stop the feeding of said recording sheet,when said electricity measured by said measuring device does not fallwithin said permission range.
 9. An image-fixing apparatus according toclaim 1, wherein said control device comprises:control means forcontrolling said supplying of said electricity from said supplyingdevice to said heater roller, so that said electricity measured by saidmeasuring device falls within a control range fully covered by saidpermission range; and stopping means for stopping said supplying of saidelectricity from said supplying device to said heater roller when thecontrolling of said control means regarding said supplying of saidelectricity is not effective in causing said electricity measured bysaid measuring device to remain within said control range and eventuallythe measured electricity has deviated from said permission range.
 10. Animage-fixing apparatus according to claim 1, wherein said measuringdevice measures at least one of an electric current and an electricvoltage of said electricity supplied from said supplying device to saidheater roller, said memory storing said relationship between said atleast one of said width and said thickness of said recording sheet andsaid permission range regarding said at least one of said electriccurrent and voltage of said electricity.
 11. An image-fixing apparatuscomprising:a presser roller which is rotatable about a first rotationaxis thereof; a heater roller which is rotatable about a second rotationaxis thereof parallel to said first rotation axis and which is held incontact with said presser roller to provide a nipper for nipping arecording sheet bearing an unfixed image thereon, an electric resistanceof said heater roller decreasing as respective contact areas of saidheater and presser rollers which areas define said nipper increasebecause of elastic deformation of the heater roller; a supplying devicewhich supplies a electricity to said heater roller so that said nippergenerates a thermal energy to thermally fix said unfixed image on saidrecording sheet; a measuring device which measures said electricitysupplied from said supplying device to said heater roller; a firstsheet-data obtaining device which obtains first sheet data representingat least one thermal parameter of said recording sheet; a first memoryin which is stored a first relationship between said at least onethermal parameter of said recording sheet and a control range regardingsaid electricity; and a control device which controls the supplying ofsaid electricity from said supplying device to said heater roller, sothat said electricity measured by said measuring device falls withinsaid control range corresponding to said at least one thermal parameterof said recording sheet represented by said first sheet data obtained bysaid first sheet-data obtaining device.
 12. An image-fixing apparatusaccording to claim 11, wherein said heater roller comprises:(a) acylindrical resilient layer which is formed of an electricallyinsulating and resilient material, is concentric with said secondrotation axis, and has a plurality of through holes formed through athickness thereof; (b) a first cylindrical electrode layer which isconcentric with said second rotation axis and has a plurality ofelectrodes each of which extends into a corresponding one of saidthrough holes of said resilient layer; (c) a cylindrical resistant layerconcentric with said second rotation axis, said resistant layer and saidfirst electrode layer sandwiching said resilient layer; and (d) a secondcylindrical electrode layer concentric with said second rotation axis,said second electrode layer and said resilient layer sandwiching saidresistant layer,a portion of said resilient layer corresponding to saidnipper being resiliently deformable so that a portion of said firstelectrode layer corresponding to the nipper is held in electric contactvia the electrodes thereof corresponding to the nipper with a portion ofsaid resistant layer corresponding to the nipper, said supplying devicesupplyimg said electricity to said heater roller through said first andsecond electrode layers so that said portion of said resistant layercorresponding to said nipper generates said thermal energy to thermallyfix said unfixed image on said recording sheet.
 13. An image-fixingapparatus according to claim 11, wherein said first sheet-data obtainingdevice comprises a sheet-parameter measuring device which measures, assaid first sheet data, said at least one thermal parameter of saidrecording sheet.
 14. An image-fixing apparatus according to claim 13,wherein said sheet-parameter measuring device measures, as said at leastone thermal parameter of said recording sheet, at least one of a width,a thickness, a surface roughness, and a moisture content of saidrecording sheet.
 15. An image-fixing apparatus according to claim 13,wherein said first sheet-data obtaining device comprises an input devicewhich is operable for inputting said first sheet data representing saidat least one thermal parameter of said recording sheet.
 16. Animage-fixing apparatus according to claim 11, wherein said first memorystores, as said first relationship, a first data table including aplurality of control ranges corresponding to a plurality of values ofsaid at least one thermal parameter of said recording sheet.
 17. Animage-fixing apparatus according to claim 16, wherein said controldevice comprises:first selecting means for selecting, from said firstdata table stored in said first memory, one of said control rangescorresponding to said at least one thermal parameter of said recordingsheet represented by said first sheet data obtained by said firstsheet-data obtaining device; and control means for controlling saidsupplying of said electricity from said supplying device to said heaterroller, so that said electricity measured by said measuring device fallswithin the selected one control range.
 18. An image-fixing apparatusaccording to claim 16, further comprising:a second sheet-data obtainingdevice which obtains second sheet data representing at least one of awidth and a thickness of said recording sheet; and a second memory inwhich is stored a second relationship between said at least one of saidwidth and said thickness of said recording sheet and a permission rangeregarding said electricity.
 19. An image-fixing apparatus according toclaim 18, wherein said second memory stores, as said secondrelationship, a second data table including a plurality of permissionranges corresponding to a plurality of values of said at least one ofsaid width and said thickness of said recording sheet.
 20. Animage-fixing apparatus according to claim 19, wherein said controldevice comprises:second selecting means for selecting, from said seconddata table stored in said second memory, one of said permission rangescorresponding to said at least one of said width and said thickness ofsaid recording sheet represented by said second sheet data obtained bysaid second sheet-data obtaining device; and stopping means for stoppingsaid supplying of said electricity from said supplying device to saidheater roller, when said electricity measured by said measuring devicedoes not fall within the selected one permission range.
 21. Animage-fixing apparatus according to claim 11, wherein said controldevice comprises adjusting means for decreasing said electricitysupplied from said supplying device to said heater roller, by a firstunit amount, when said electricity measured by said measuring device isgreater than an upper limit of said selected one control range, andincreasing said electricity supplied from said supplying device to saidheater roller, by a second unit amount, when said electricity measuredby said measuring device is smaller than a lower limit of said selectedone control range.
 22. An image-fixing apparatus according to claim 11,wherein said control device comprises:a control-amount data obtainingdevice which obtains control-amount data representing a difference of(a) a first value of said electricity which is taken as a result ofoperation of said control device and (b) a second value of saidelectricity which would otherwise be taken without said operation ofsaid control device; and stopping means for stopping said supplying ofsaid electricity from said supplying device to said heater roller whensaid difference between said first and second values of said electricitydoes not fall within a reference control-amount range.
 23. Animage-fixing apparatus according to claim 22, wherein said controldevice further comprises adjusting means for decreasing said electricitysupplied from said supplying device to said heater roller, by a firstamount, when said electricity measured by said measuring device isgreater than an upper limit of said control range, and increasing saidelectricity supplied from said supplying device to said heater roller,by a second amount, when said electricity measured by said measuringdevice is smaller than a lower limit of said control range,saidcontrol-amount data obtaining device obtaining said control-amount datarepresenting, as said difference, a difference of said first and secondamounts, said stopping means stopping said supplying of said electricityfrom said supplying device to said heater roller when said difference ofsaid first and second amounts does not fall within said referencecontrol-amount range.
 24. An image-fixing apparatus according to claim23, wherein said measuring device measures at least one of an electriccurrent and an electric voltage of said electricity supplied from saidsupplying device to said heater roller, said first memory storing saidfirst relationship between said at least one parameter of said recordingsheet and said control range regarding said at least one of saidelectric current and voltage of said electricity.
 25. An image-fixingapparatus according to claim 22, wherein said control device furthercomprises adjusting means for decreasing one of an electric current andan electric voltage of said electricity supplied from said supplyingdevice to said heater roller, by decreasing the other of said electriccurrent and voltage by a first amount, when said electricity measured bysaid measuring device is greater than an upper limit of said controlrange, and increasing said one of the electric current and voltage byincreasing said other of the electric current and voltage by a secondamount, when said electricity measured by said measuring device issmaller than a lower limit of said control range,said control-amountdata obtaining device obtaining said control-amount data representing,as said difference, a difference of said first and second amounts, saidstopping means stopping said supplying of said electricity from saidsupplying device to said heater roller when said difference of saidfirst and second amounts does not fall within said referencecontrol-amount range.
 26. An image-fixing apparatus according to claim25, wherein said measuring device as a first measuring device measuressaid one of said electric current and voltage of said electricitysupplied from said supplying device to said heater roller, saidcontrol-amount data obtaining device comprising a second measuringdevice which measures said other of said electric current and voltage ofsaid electricity supplied from said supplying device to said heaterroller, said first memory storing said first relationship between saidat least one parameter of said recording sheet and said control rangeregarding said one of said electric current and voltage of saidelectricity.
 27. An image-fixing apparatus comprising:a presser rollerwhich is rotatable about a first rotation axis thereof; a heater rollerwhich is rotatable about a second rotation axis thereof parallel to saidfirst rotation axis and which is held in contact with said presserroller to provide a nipper for nipping a recording sheet bearing anunfixed image thereon, an electric resistance of said heater rollerdecreasing as respective contact areas of said heater and presserrollers which areas define said nipper increase; a supplying devicewhich supplies an electricity to said heater roller so that said nippergenerates a thermal energy to thermally fix said unfixed image on saidrecording sheet; a measuring device which measures said electricitysupplied from said supplying device to said heater roller; and asheet-jamming detecting device which detects a jamming of said recordingsheet at said nipper, based on said electricity measured by saidmeasuring device.
 28. An image-fixing apparatus according to claim 27,said sheet-jamming detecting device comprises means for identifying thatsaid jamming of said recording sheet has occurred, when said electricitymeasured by said measuring device does not fall within a referencerange.
 29. An image-fixing apparatus according to claim 27, saidsheet-jamming detecting device comprises means for identifying that saidjamming of said recording sheet has occurred, when a rate of change ofsaid electricity measured by said measuring device does not fall withina reference range.
 30. An image-fixing apparatus according to claim 27,further comprising an informing device which informs that saidsheet-jamming detecting device has detected said jamming of saidrecording sheet.
 31. An image-fixing apparatus according to claim 27,further comprising a control device which stops the supplying of saidelectricity from said supplying device to said heater roller, when saidsheet-jamming detecting device has detected said jamming of saidrecording sheet.